1. Field of the Invention
The present invention relates to color displays using LCDs. More particularly, the present invention relates to sequential color displays which use a novel control system for ferroelectric LCDs.
2. Description of the Related Art
In color displays, there are three major systems for producing different colors and brightness of colors. In the first, a number of pixels are provided, each pixel transmitting either red, green, or blue light. The pixels are arranged in groups of red, green, and blue. A particular color is achieved in an area by turning off or on the appropriate pixels in that area. For example, if purple is the desired color in an area, the green pixels in that area would remain off and the red and blue pixels would be turned on. The brightness is also controlled by turning on or off pixels. If bright purple in an area is desired, then all of the red and blue pixels would be turned on in that area. If a darker purple is desired, then some red and blue pixels would remain off in that area.
In a second system, similar to the first, a number of pixels are also provided, each pixel transmitting either red, green, or blue light. The pixels are again arranged in groups of red, green, and blue, and again, a particular color is achieved in an area by turning off or on the appropriate pixels in that area. However, brightness is controlled by varying the amount of light being transmitted by a pixel which is on, rather than turning off some of the pixels. As in the first system, if bright purple in an area is desired, then all of the red and blue pixels would be turned on in that area. If a darker purple is desired in an area, then rather than having some of the red and blue pixels remain off in that area, all of the red and blue pixels transmit light, but the amount of light being transmitted from each pixel varies. This second system allows for higher resolution than the first system.
Baur, et al., in "High performance liquid crystal device suitable for projection display," SPIE Proceedings, Volume 2650, pages 226-228 (Jan. 29-31, 1996) (incorporated herein by reference), disclose a system similar to this second type in which liquid crystals are used as light valves to alter the polarization of incident light on pixels such that more or less of the light striking the pixels will ultimately be transmitted to a display through a beamsplitter. U.S. Pat. No. 5,453,859 (incorporated herein by reference), issued to Sannohe, et al., discloses another similar system.
A third system for producing a color display with various colors and brightness of colors is commonly known as field sequential color. In a field sequential color system, each pixel transmits, sequentially in time, red, green, and blue light. When the transmission is fast enough, the human brain fuses all three colors of light into a single color, which is a blend of the colors. Color and brightness of color can be controlled in the time domain. For example, if a bluish, purple color is desired from the pixel during a certain time period, the pixel will transmit blue light longer than red light, and it will transmit no green light. Field sequential color is advantageous in that it allows for very high resolution, since each pixel is independent of its neighbors and can assume any color. However, it has limitations which make it a challenge to commercially exploit, including a requirement for extremely high switching rates. This is in part needed to reduce certain undesirable color effects, including rainbows and color flashes associated with moving objects.
LCDs can be used to valve the light transmitted from a pixel in a color sequential system. The LCD must operate at triple the frame rate, because red, green, and blue are done sequentially. Ferroelectric LCDs can do this, but they are binary (on/off), not analog devices, and so they cannot do gray scales with an analog signal input.
DisplayTech, Inc., in a 6-page technical disclosure entitled "FLC/VLSI Display Technology" and dated Dec. 1, 1995 (incorporated herein by reference), discloses a field sequential color system using ferroelectric liquid crystals. In that DisplayTech system, a spatial light modulator (SLM) consists of an array of square pixels organized in rows and columns. Each pixel in the array has a light valve on top of a reflector, and the light valve can be turned on (allowing the light valve to transmit light to the reflector), or the light valve can be turned off (causing the pixel to not transmit light, and thus appear black). The SLM is a very large scale integrated (VLSI) chip that can be a specialized version of a complementary metal oxide semiconductor (CMOS) dynamic random access memory (DRAM) in which each memory cell corresponds to a pixel. The VLSI chip is described as being less than one inch in diagonal size. The light valve is described as a very high speed (100 microsecond switching time) ferroelectric liquid crystal (FLC) in contact with the VLSI chip and which is formed into a thin layer trapped by an overlying window which passes visible light.
By electrically writing image data bits to the DRAM memory cells, images are displayed with the DisplayTech system. The stored charge in each cell impresses a voltage across the FLC to turn it off or on, thus causing light to be transmitted or not from the pixel which the FLC controls.
In the DisplayTech system, color and gray scale is determined in the time domain. The DisplayTech paper discloses that DisplayTech is able to achieve 5-bit gray (31 levels of brightness) by turning the FLC to be on for between zero and 31 time units. However, the DisplayTech paper does not establish how this time is to be established; it appears that each pixel has 15 time periods per frame where it is off or on, and that the FLC can be switched on or off 15 times per frame (5 time periods per color, each time period lasting 1, 2, 4, 8, or 16 time units).
The DisplayTech paper mentions that the basic operation of its FLC/VLSI display requires a relatively large bandwidth of the input video signal which transfers information from the signal source (larger than for analog approaches, it points out).
Bright light sources can be used advantageously in field sequential color systems. One type of light source which is suitable for use in field sequential color systems is a lamp known as a high intensity discharge (HID) lamp. A HID lamp consists of a glass envelope which contains electrodes and a fill which vaporizes and becomes a gas when the lamp is operated. U.S. Pat. No. 5,404,076 (incorporated herein by reference), issued to Dolan, et al., and entitled "Lamp Including Sulfur" discloses an electrodeless lamp utilizing a fill containing sulfur or selenium or compounds of these substances at a pressure at least as high as one atmosphere. The fill is excited at a power density in excess of 50 watts per square centimeter. An arc lamp utilizing the fill at a pressure at least as high as one atmosphere is excited at a power density of at least 60 watts per square centimeter. This lamp could advantageously be used with field sequential color systems.
Parfenov, et al., in "Advanced optical schemes with liquid crystal image converters for display applications," SPIE Proceedings, Volume 2650, pages 173-179 (Jan. 29-31, 1996--incorporated herein by reference), disclose additional background information on the use of liquid crystal devices to process video images.